Sintered anisotropic permanent magnet



SINTERED ANISQTRQPI'C PERMANENT MAGNET Robert J. Studdc'rs, Seltzeneetaily, Ns'Y, assignor to General Electric Company, a corporation of New York No Drawing. Application February 17, 1948, Serial No. 9,013

3 Claims. (Cl.-317-202) The present invention relates to sintered anisotropic permanent magnet alloys. with sintered Alnico magnets prepared by powder metallurgy techniques and having BH max. values corresponding to or approaching those of the best cast Alnico magnets.

Essentially isotropic permanent magnets of the type known as Alnico magnets contain iron, nickel and aluminum as basic or essential ingredients. It is well known or has been proposed to include other elements such as cobalt, copper, titanium, silicon, chromium, molybdenum, tungsten, manganese, sulphur, and others either for the purpose of modifying or enhancing the physical and/or magnetic properties of the alloys. Many of these magnetic alloys have been produced in both cast and sintered form, and provided the proper procedures are followed in the manufacture of the sintered products to prevent oxidation of the aluminum and any other highly oxidizable elements, it has been found that for most of the alloys the sintered products exhibited the same or approximately the same magnetic properties as the cast products of the same composition.

Another known development in the Alnico magnet field was the discovery described more fully in Jonas Patent 2,295,082 that certain cast Ni-Al-Fe alloys having a cobalt content of 16 to 30%, a nickel content of 12 to 20%, and an aluminum content of 6 to 11% when heat treated in a magnetic field would exhibit BH max. values in one direction (anisotrope) which was at least 50%, and generally over 100%, higher than that for the same alloy whose magnetic properties were substantially equal in all directions (isotrope). As a result of this development there is now commercially available cast Alnico magnets having approximately the following minimum magnetic properties:

Peak H oersteds 2,000 Peak B gauss 15,700 Coercive force He oersteds 575 Residual Br gauss 12,000 BH max gauss-oersteds 4.5 10

A cast magnet possessing these excellent magnetic values consists of an alloy of about 8% aluminum, 14% nickel, 24% cobalt, 3% copper, balance substantially iron, the exceptional magnetic properties being obtained .by heat treatment or cooling of the alloy in a magnetic field as described in the above-mentioned Jonas patent. Because of the exceptional magnetic qualities of this cast alloy and the known advantages of the powder metallurgy techniques numerous attempts have been made to duplicate its magnetic properties in a sintered magnet alloy.

Unlike the isotropic type of Alnico alloys, it was found that simple duplication of the chemical composition of the anisotropic type of alloy, as described in the Jonas patent, in a sintered product, did not result in a very satisfactory permanent magnet. There appears to be many other factors which influence the effectiveness of the magnetic field treatment in imparting superior available energy products to the alloy.

The present invention is based on the discovery that an anisotropic sintered magnet can be prepared by including additions of titanium in the compositions now employed in making cast magnets having directional properties. It has been found that the titanium is an It is particularly concerned I 2,694,790 Patented Nov. 16, 1954 essential element of the sintered magnets of the present invention in order to obtain a high available energy product. In accordance with prior teachings concerning the effect of titanium in the cast magnets this element has been considered non-essential and when employed, its effect was that of a lattice stress raiser with a tendency to increase the coercive force of the cast alloy at some sacrifice of available energy. In the sintered products of the present invention the titanium addition does not produce as marked an increase in coercive force as noted with the cast alloys. The titanium addition in the sintered product appears to make the magnetic field treatment more effective in obtaining a high available energy product, which product is an accepted criterion for comparing permanent magnet alloys. Hence, the effect of the titanium on the magnetic properties seems to be quite different in the sintered and cast alloy fields.

The sintered magnets of the present invention are characterized by a titanium content of at least 0.25% and not over 1.5%, generally from 0.6 to 1.25%. While the invention is broadly applicable to the various known anisotropic magnet alloys characterized by a cobalt content of 16 to 30%, a nickel content of 11 to 20%, and an aluminum content of 6 to 11%, the preferred sintered magnets are those containing 7.5 to 9% aluminum, 0.6 to 1.25% titanium, 22 to 26% cobalt, 2 to 4% copper, 13 to 15% nickel, remainder iron except for incidental impurities.

Particularly good results have been obtained by employing 8.5% aluminum, 1% titanium, 25% cobalt, 3% copper, 14% nickel, balance iron. The sintered products are prepared in the manner usually employed in making sintered Alnico magnets as described for example in Howe Patents 2,192,743 and 2,192,744. A foundation alloy of iron-aluminum, cobalt-aluminum or nickel-aluminum is used as the source of aluminum, while a titanium alloy such as a 70-30 titanium-nickel alloy can be used to supply the desired titanium content of the product. The finely-divided materials are mixed in the desired proportions, and pressed to the desired form or shape. The pressed products are sintered in a hydrogen atmosphere at temperatures of from 1000 to 1400 C. preferably at a temperature below but close to the melting point of the alloy. The time required for the sintering action will, of course, depend upon the furnace load and size of the pieces to be sintered. The sintered material can then be normalized by heating to an elevated temperature after which the sintered product is subjected to a further heat treatment in a magnetic field as described in the Jonas Patent 2,295,082 to make the sintered alloy magnetically anisotropic. The heat treatment in the magnetic field is preferably carried out by withdrawing the sintered alloy compacts from the sintering zone of the furnace at a temperature of about 1250 C. and controlling their cooling cycle in a magnetic field of proper field strength. Further low temperature treatments may be applied as described by onas.

The resultant magnets are characterized by a BH max. at least equal to 3.5 10 gauss-oersteds and in some cases energy products as high as 4.5)(10 have been obtained. In general, the magnetic properties of the sintered magnets of the specific formula given hereinbefore include a peak B of at least 15,000, a Br of at least 10,000, an Hc of at least 600 and a BH max. of about 4 to 4.25 X10 These BH max. values are to be compared with values of from 1.4 to 1.6)(10 for the best commercially available sintered Alnico magnets and 4.5 to 6x10 for the best anisotropic cast Alnicos.

As indicated hereinbefore, the effect of titanium on the sintered products difiers from that in the cast alloys. For instance, whereas the addition of one per cent titanium to the best anisotropic cast Alnico, i. e., an Alnico containing 8% aluminum, 14% nickel, 24% cobalt, 3% copper, balance iron lowers the BH max. value of the alloy, the addition of the same amount of titanium to a sintered Alnico composition of the same composition increases the BH max. value.

This eiiect of titanium on the magnetic properties of cast and sintered Alnico products is apparent from the following table setting forth the representative magnetic 3 values of cast and sintered products all of which contained from 8 to 8.5% aluminum, 24% cobalt, 14 to 14.5% nickel, to 3.25% copper, balance iron except in those cases where 1% titanium has been substituted for a corresponding amount of iron:

Titanium Alloy B, H. BH max None Cast 12,500-13,000 540-600 4.5 to 5.5x10 None Sintered 11,000-12,000 450-550 2.0m 3.0)(10 1% Cast 10,000-10,500 750-800 2.8 to 3.5 10 1% Sintered... 10,000-10,500 600-650 3.5 to 4.5X10

What I claim as new and desire to secure by Letters Patent of the Un1ted States, 1s:

1. An anisotropic slntered permanent magnet containmg 7.5 to 9% aluminum, 13 to 15% nickel, 22 to 26% cobalt, 2 to 4% copper, 0.6 to 1.50% titanlum, balance iron except for incidental unpurities, said magnet having a BH max. in the principal direction at least equal to 3.5 10

2. An anisotropic sintered permanent magnet consisting of 8.5% aluminum, 14% nickel, 25% cobalt, 3%

copper, 0. 6-1.25% titanium, balance iron except for incidental impurities, said smtered magnet having a BH max. of at least 3.5 X 10 3. An anisotropic sintered permanent magnet consist- 5 ing of 8.5% aluminum, 14% nickel, 25% cobalt, 3% copper, 1% titanium, balance substantially all iron, said magnet having a BH max. of at least 3.5X10

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,167,240 Hensel July 25, 1939 2,245,477 Jonas June 10, 1941 15 2,285,406 Bieber June 9, 1942 2,295,082 Jonas Sept. 8, 1942 2,384,450 Bieber Sept. 11, 1945 FOREIGN PATENTS 20 Number Country Date 102,215 Great Britain Oct. 6, 1937 462,248 Great Britain Mar. 4, 1937 522,731 Great Britain June 26, 1940 

1. AN ANISOTROPIC SINTERED PERMANENT MAGNET CONTAINING 7.5 TO 9% ALUMINUM, 13 TO 15% NICKEL, 22 TO 26% COBALT, 2 TO 4% COPPER, 0.6 TO 1.50% TITANIUM, BALANCE IRON EXCEPT FOR INCIDENTAL IMPURITIES, SAID MAGNET HAVING A BH MAX. IN THE PRINCIPAL DIRECTION AT LEAST EQUAL TO 3.5X106. 